單分子導電裝置裡的量子輸送現象

我的碩士論文由三個不同主題的章節組成，目的是探討單分子導電裝置的各個不同面向。在第一章裡，我利用兩原子佩爾斯-賀伯特模型(two-site Peierls-Hubbard model)觀察混價分子的導電行為，並討論電子排斥力與分子振動在其間的影響。由於這兩作用力之間的競爭導致此模型的哈密爾敦函數(model Hamiltonian)的結構不穩定性，且直接地影響了其輸送行為，因此可以將此系統的輸送圖譜做出分類而能與分子的絕熱位能面間的關係聯繫起來。我更進一步指出由於分子的振動位能面的扭曲可能使圖譜上部分的庫倫鑽石(Coulomb diamonds)消失，而且在單重態與三重態之間的自旋翻轉(spin flip)可能導致負電阻(negative differential conductance)。在第二章裡，我研究以單一混價雙原子分子為主體的分子電晶體在弱作用力極限(weak coupling limit)下的熱電效應的特徵。以兩原子佩爾斯-賀伯特模型(two-site Peierls-Hubbard model)模擬分子的行為，而此模型的哈密爾敦函數(model Hamiltonian)會由於本身的作用力之間的競爭而使其絕熱位能面呈現出不同的行為。於此我專注在庫倫堵塞振盪區域(Coulomb-blockade oscillation regime)且討論熱電效應是如何呈現受到分子的震動而有不同的對稱性的分子激發態的特徵。在最後一章裡，我論述一個理想的分子導線不但需要極佳的導電度，也需要一層絕緣體的保護。我在這裡討論為何長鏈金屬串分子是一個可以做為實際應用的好的選擇。電子通過其中間的金屬原子進行傳遞，並受到周圍的有機共軛官能基團保護而達到絕緣的效用。我也進一步指出在這樣一個量子迴路中會因為電極的長距離作用力而在每一個原子上有隱藏的通路直接與電極傳遞。但是在經過來自電極的自我能(self energy)的修正後，克西何夫定律(Kirchhoff's junction rule)依舊是成立的。

關鍵字

量子輸送；佩爾斯-賀伯特模型；熱電效應；長鏈金屬串分子

並列摘要

My thesis is composed of three chapters which address several different but related aspects of the quantum transport in single molecular electronic devices. In the first chapter, we investigate the relative importance of electron-electron and electron-phonon interactions on conductive behaviors of a mixed-valence molecule by using the two-site Peierls-Hubbard model. Transport behaviors depend on the interplay among these two interactions due to the structural instability that exhibits in this model Hamiltonian. We also demonstrate that the transport spectrum of this system can be qualitatively classified according to different types of adiabatic potential profiles. Moreover, we show that the distortion of the adiabatic potential surface may result in the missing of Coulomb diamonds and the negative differential conductance arises from the inelastic tunneling or the spin flip encountered between singlet and triplet states. In the second chapter, we study the characteristics of thermopower of a molecular transistor consists of a single mixed-valence dimer in the weak coupling limit. The molecule is modeled by the two-site Peierls-Hubbard model, and its adiabatic potential profiles can exhibit different broken symmetries due to the competition of molecular parameters. In this chapter, we focus on the Coulomb-blockade oscillation regime and show how the thermopower reflects the symmetry of the first excited state which directly corresponds to different electron-phonon coupling. Finally, in the final chapter, we argue that an ideal electrical wire needs not only good conductivity for its central conductor but also a surrounding insulating layer in order to protect its current from leaking. We show that compounds consisted of the extended metal-atom chains are promising candidates to be the smallest molecular electrical wire for future practical applications. The electron can tunnel across core metals easily, while the internal current is insulated from outside by the surrounding $pi$-conjugated functional groups. Moreover, we also show the existence of unavoidable hidden pathways at each site to the electrodes in a nanoscaled quantum circuit. Nevertheless, the Kirchhoff's junction rule still holds when the current inflow and outflow arising from the additional terms of the self energies of contacts are included.

並列關鍵字

quantum transport ； Peierls-Hubbard model ； thermopower ； extended metal atom chain

參考文獻

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單分子導電裝置裡的量子輸送現象

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